1. Field of the Invention
The present invention relates to a lithium secondary battery comprising a cathode having a lithium cobalt complex oxide or a lithium nickel complex oxide as a cathode active material.
2. Related Art of the Invention
Portable and cordless electronic appliances are becoming increasingly popular. As the power source for such electronic appliances, there is a great demand for small-sized and lightweight secondary batteries having a high energy-density. Lithium secondary batteries comprising a non-aqueous electrolytic solution have been put to practical use to meet such demands.
In general, a lithium secondary battery consists essentially of a cathode having a lithium-containing compound as the active material, an anode having a material capable of absorbing and desorbing lithium, such as a carbon material, or lithium metal as the active material, a separator, and a non-aqueous electrolytic solution. Of such elements constituting the battery, the lithium-containing compound for the cathode active material may include, for example, lithium cobalt complex oxides (LiCoO.sub.2, etc.), lithium nickel complex oxides (LiNiO.sub.2, etc.), and lithium manganese complex oxides (LiMn.sub.2 O.sub.4, etc.). Of these, LiCoO.sub.2 and LiNiO.sub.2 are now in practical use. The LiNiO.sub.2 produces a larger depth of charge and discharge to give a larger discharge capacity compared with LiCoO.sub.2 and the cost of the source of LiNiO.sub.2 is lower than that of LiCoO.sub.2.
In conventional lithium secondary batteries comprising the lithium cobalt complex oxide (LiCoO.sub.2) or the lithium nickel complex oxide (LiNiO.sub.2) as the cathode active material, particles of LiCoO.sub.2 or LiNiO.sub.2 having a large particle size and a small specific surface area are used in order to increase the density of the cathode active material to thereby improve the utilization of the batteries, or in order to prevent the decomposition of the electrolytic solution in the batteries. However, the conventional batteries comprising such large particles of LiCoO.sub.2 or LiNiO.sub.2 are problematic in that they require shallow charge and discharge. This is because when the batteries repeatedly undergo deep charge and discharge, their cycle life is much shortened. In a shallow charge and discharge, however, Li ions diffuse in the cathode active material insufficiently, resulting in that the energy density per the unit volume of the batteries being lowered.
On the other hand, lithium secondary batteries comprising particles of LiCoO.sub.2 or LiNiO.sub.2 having a small particle size and a large specific surface area are also problematic in that the electrolytic solution that is in contact with the small particles of LiCoO.sub.2 or LiNiO.sub.2 is decomposed whereby the charge-discharge cycle characteristics and also the storage characteristics of the batteries are worsened. In addition, since such small particles of LiCoO.sub.2 or LiNiO.sub.2 are difficult to shape, there is still another problem with these in that LiCoO.sub.2 or LiNiO.sub.2 of that type requires a large amount of binder when shaped into cathodes, thereby lowering the density of the cathode active material and lowering the energy density per the unit volume of the batteries.
Given the situation, the object of the present invention is to solve the above-mentioned problems and to provide a lithium secondary battery having high capacity and having excellent charge-discharge cycle characteristics.